The present invention relates to improvements for a plug used to form a seal for tubes, pipes and similar conduits which transport steam, liquid or gases under pressure.
An earlier version of this type of plug commercially exploited the present Applicant is disclosed in U.S. Pat. No. 4,653,540. The plug disclosed in that patent comprises a hollow housing member which has at one end a relatively large open bore surrounded by a thin wall, and a relatively smaller threaded bore which communicates with the larger bore. An insert member is provided which, in use, is threaded into the smaller bore thereby causing a tapered portion which is formed on the insert member to engage the open end of the large bore causing it to expand and bear against the wall of a heat exchanger tube into which the heat exchanger tube plug has been inserted. During use, the housing member is prevented from rotating relative to the heat exchanger tube by means of an eccentric assembly which is mounted on the end of the housing member. At the other end, the insert member has a cylindrical portion and a centrally disposed, integrally formed nut portion. The nut is proportioned to be grasped by a wrench for screwing the insert member into the housing member. The disclosure also suggests that instead of the projecting nut portion, a hexagonal recess may be formed in the end of the cylindrical portion for insertion of a hex wrench or similar tool. The insert member is specified as being integrally formed. A coil spring 34 surrounds the eccentric ring 36 to provide locking engagement against the inner surface of the heat exchanger tube.
The clutch spring on the eccentric ring was abandoned in the production model of this plug. Instead, a knurled outer surface for the eccentric ring and also short axial grooves on the plug body were added in order to prevent slipping when the eccentric cam actuated.
Also, annular grooves were added to the plug body to increase the sealing potential of the plug. If the plug was installed in a tube that had corrosion, then the raised portion of the serration would cut through any scale and would be stored in the groove thereby enhancing the mechanical contact seal.
Early on during manufacture of the plug defined in U.S. Pat. No. 4,653,540, the plugs had a machined male hex that would accommodate a 11/32 inch socket. This, however, proved unsatisfactory because the sockets split and could not be turned down to a diameter in order to fit in any tube smaller than 1/2 inch. This would be necessary when the plug is recessed within the end of a tube to be sealed. The next design evolution entailed broaching a 1/4 inch female square drive in plugs from 460 inches to 0.610 inches in diameter and a 3/8 inch square drive in all plugs 0.620 inches and larger.
The shortcomings of this female square drive have included that the smallest plug that can be manufactured is 0.460 inches in diameter. The heat exchanger industry has vessels with 158 inches by 12 gauge diameter tubes (0.420 inch nominal i.d.) for which plugs having a 0.460 inch diameter cannot be used. A 1/4 inch square drive has a diagonal dimension of 0.354 inches leaving only 0.53 inches of material at the corners of the broach if a 0.460 inch diameter plug is used. This thin amount of material at the corners of the broach results in cracking in the corners and ultimate failures of the plug when installed at a recommended torque value of 400 inch pounds.
Also, commercially available tooling in 1/4 inch socket drive extensions can withstand only 425 to 460 inch pounds of torsional force before failure. This has limited the recommendation for installation to a maximum of 400 inch pounds in order to provide a safety factor. In reality, 600 to 650 inch pounds of torque, as discerned from testing, would yield a higher radial expansion force and a better sealing capability by as much as 50%.
It therefore would be an improvement in this art if a tube plug were provided which permitted a higher torque level for installation and such a plug which could be used in smaller diameter sizes.